Structural, Magnetic, Electrical, and Magnetoelectric Properties of Sm- and Ho-Substituted Nickel Ferrites

Sm- and Ho-substituted and pure nickel ferrite materials, namely, NiFe1.925Sm0.075O4, NiFe1.925Ho0.075O4, and NiFe2O4, have been synthesized by the solid-state chemical reaction, and their structural, magnetic, dc electrical conductivity, ferromagnetic, ferroelectric, and dielectric properties have been evaluated. Sm- and Ho-substituted nickel ferrites crystallize in the cubic inverse spinel phase with a very small amount of SmFeO3 and HoFeO3 as the additional phase, respectively. X-ray diffraction studies indicate rhombohedral distortion in NiFe1.925Sm0.075O4 and NiFe1.925Ho0.075O4 compounds. The existence of ferroelectricity in NiFe1.925Sm0.075O4 and NiFe1.925Ho0.075O4 has been confirmed from the ferroelectric loops, and the respective transition temperatures are 543 and 677 K. The respective magnetocapacitance values are −1.8% and −1.2%. Magnetoelectric coefficients observed at 500 Oe in the NiFe1.925Sm0.075O4 and NiFe1.925Ho0.075O4 compounds are 1.82 and 1.84 mV cm−1 Oe−1, respectively. A considerable increase in the saturation magnetostriction value has been observed upon the substitution of Sm. Substitution of Sm and Ho for Fe at the B site increases the dielectric constant compared to that of pure nickel ferrite. Frequency variation of the dielectric constant shows a dispersion that can be modeled with a modified Debye function, which considers the possibility of more than one ion contributing to the relaxation. Electrical conductivity curves confirm the improved resistivity of the NiFe1.925Sm0.075O4 and NiFe1.925Ho0.075O4 compounds compared to that of pure nickel ferrite. Analysis of the temperature-dependent conductivity indicates that the small polaron and variable-range-hopping mechanisms are operative in the 220−300 and 160−220 K temperature regions, respectively.